Single sheet thermoforming processes typically involve providing a preformed sheet of thermoplastic material (usually on a roll), heating the preformed thermoplastic sheet to a thermoformable temperature, and contacting the heated thermoplastic sheet with a shaped mold surface. The heated thermoplastic sheet is typically drawn into contact with the shaped mold surface by means of a vacuum being drawn through perforations in the mold surface.
Such prior art methods of single sheet thermoforming typically and undesirably involve multiple steps, such as separately forming the thermoplastic sheet, collecting the preformed thermoplastic sheet on a roll, transporting (e.g., shipping) the roll of preformed thermoplastic sheet to a molder (or fabricator), and re-heating the performed thermoplastic sheet prior to the thermoforming operation. In addition, such prior art methods of single sheet thermoforming also typically do not provide sufficient control of the thermoplastic sheet thickness over contoured (e.g., recessed) mold surfaces. For example, the molded thermoplastic sheet may be too thin (e.g., in deep drawn areas), and/or too thick in other areas. Such variability in molded sheet thickness may result in the final molded article having undesirably variable physical properties, such as crack failure, and aesthetic properties, such as an uneven visual appearance (e.g., irregular color).
Thermoforming processes that involve the continuous extrusion of a thermoplastic sheet, that is thermoformed using residual heat from the extruded thermoplastic sheet are known. See, for example, U.S. Pat. Nos. 6,814,905 B1, 6,086,800 and 4,061,706. Such continuous thermoforming methods, while perhaps addressing or eliminating some of the multiple steps involved with the use of preformed thermoplastic sheets, typically and undesirably do not provide sufficient control of the thermoplastic sheet thickness over contoured (e.g., recessed) mold surfaces.
U.S. Pat. No. 4,555,377 discloses an indexed thermoforming machine having a cold sheet loading station, sheet heating stations, a molding station, and a clamp frame carrier assembly. The clamp frame carrier assembly of the '377 patent is described as including pivotally operated clamps that control sag in a clamped thermoplastic sheet after it is heated and before it is molded.
It would be desirable to develop new thermoforming processes, and apparatuses used therewith, that minimize or eliminate the steps typically encountered with prior art methods. In addition, it would be further desirable that such newly developed methods and apparatuses also provide improved control of the thickness of the molded thermoplastic sheet as it is formed and molded over contoured mold surfaces.